The Asian southwest monsoon (SWM) east of the Horn of Africa (HOA) is important in planning and conducting maritime operations in this region. Skillful subseasonal to seasonal (S2S) forecasts of the SWM onset are needed for operational planning but are not yet available. We investigated the potential for S2S forecasting of the onset of SWM surface winds and ocean surface waves off HOA by analyzing global atmospheric and oceanic variables, and three onset states: early, normal, and late. We identified relationships between variables, precursor conditions for each state, and potential predictors for each state. We used the predictors to conduct 48 years of hindcasting. We assessed the skill of the hindcasts and identified two Pacific sea surface temperature (SST) predictors with a high potential to contribute to S2S forecasts of the onset. These results indicate that El Niño / La Niña (ENLN) variations are important in generating early, normal, and late onsets, but are not the only factors. The mechanisms that link remote SSTs to the onset appear to involve anomalous tropical Rossby-Kelvin waves that extend across the western tropical Pacific and the Indian Ocean–south Asian regions.

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